Currently, there are several so-called “last mile” and “last foot” data transmission systems which are designed to deliver high speed and/or high data capacity from the internet backbone to an end user. Several such systems use RF transmissions to replace copper wire or fiber optic cables. Some of these systems are called point-to-point or point-to-multipoint systems and operate in various licensed and unlicensed RF bands. A fundamental characteristic of many such existing systems is that their RF transmissions occur in a frequency spectrum protected and regulated by a government body. These protected frequency spectrums, or bands, are licensed to certain license holders and only a few may operate in any given physical area depending upon the number of licenses available.
There are only a limited number of licensed bands in any geographic area, thus, in order to widen the choices consumers have, it is desirable for service providers to be able to use unlicensed RF bands to provide high speed, high capacity data services. In addition, a service provider may not hold licenses in every geographic area that it desires to provide service. Therefore, use of the unlicensed frequency bands may allow a service provider to cover a greater geographical area.
In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (U-NII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. The FCC specifies the conditions for operating wireless equipment in the U-NII frequency band. However, operators are not protected from possible interference from other U-NII operators transmitting in the vicinity or even other systems or devices which utilize the same frequencies.
The IEEE, a standards group, is defining a wireless LAN standard, referred to as IEEE 802.11a for operation in the U-NII band. Equipment that conforms to this standard will operate indoors at the lower and middle frequency sub-bands (i.e. 5.15 to 5.25 GHz and 5.25 to 5.35 GHz), The ETSI BRAN group in Europe has defined an air interface standard for high-speed wireless LAN equipment that may operate in the U-NII frequency bands. Equipment that is compatible with this standard may cause interference with use of these unlicensed bands.
A problem that many operators face in the unlicensed bands is a need to provide the highest possible data rates to subscriber units. One prior art method of providing greater through-put in licensed bands is to have a very large channel bandwidth providing data to the subscriber. To increase through-put to a particular subscriber unit, the bandwidth must necessarily increase, or the modulation scheme that is used has to become more complicated. Problematically, bandwidth is limited in the unlicensed bands and using a higher modulation scheme causes an increase in the necessary signal to noise ratio or carrier to interference ratio (C/I). For example, increasing from a quadrature phase shift keying (QPSK) modulation to a 16 quadrature-amplitude modulation (16 QAM), which doubles the throughput, requires a 6 dB increase in C/I. Problematically, such increases in C/I may not be practical, particularly in the unlicensed bands where a significant amount of interference may be present.
Another problem associated with increasing the capacity or through-put of a RF wireless data transmission system can be the limited amount or type of hardware available to build a point-to-multipoint wireless data transmission system. For example, some chip sets are only available with a 6 Mhz channel bandwidth. This limits capacity to what can move through a 6 Mhz channel bandwidth. Likewise, to use a wider bandwidth channel, which would increase capacity, a desired modulation scheme might not be feasible, due to complications and costs in designing a system without the benefit of off-the-shelf chips.
Multiple input multiple output (MIMO) systems divide a single channel into different transmit streams, but use multiple antennas that are at the same base station location. Code division multiple access (CDMA) systems use multiple transmit codes from different base stations, using the same frequency. The global system for global communications (GSM) system combines multiple timeslots from a single frequency to increase capacity using a single antenna. Therefore, it is desirable for a wireless RF data communication system to use multiple frequencies to increase capacity.